1. Field of the Invention
The present invention is generally directed to the field of subsea oil and gas production, and, more particularly, to a lightweight device for remote subsea wireline intervention.
2. Description of the Related Art
Offshore oil and gas wells may generally be divided into two groups—surface piercing wells and subsea wells. Surface piercing wells are wells that are located on an artificial surface above sea level that is supported by a fixed structure, e.g., a platform, or a floating structure, e.g., a spar, a semi-submersible, a tension leg platform, a vessel, etc. Subsea wells reside on the sea floor, including their wellhead structure and valving control (subsea Christmas tree).
Often during the life of a well, intervention into the well bore may be required for a variety of reasons. For example, an intervention may be required to diagnose a problem, correct a problem, stimulate production and/or repair equipment within the well bore. Performing intervention operations on surface piercing wells is very straightforward as surface piercing wells are easily accessed through the top of the Christmas tree (located on the artificial surface) using traditional means developed for land-based wells, e.g., a lubricator, pressure containment assembly (wireline rams), and one or more lifting devices. Such operations can be performed at a relatively low cost due to the ready accessibility to the top of the Christmas tree on such surface piercing wells and the equipment used in performing such interventions.
However, intervention on subsea wells is much more difficult and expensive. Intervention of a subsea well frequently requires the rental and use of a surface vessel, a completion/workover riser, and both surface and subsea pressure containment assemblies (a surface tree that mimics a surface piercing Christmas tree—so that workover hardware can be attached, and a Lower Workover Riser Package (LWRP) 5—e.g. a lower riser package (LRP 5A) with actuated pressure containment rams, and an emergency disconnect package (EDP) for well control to gain surface access to the subsea Christmas tree. Equipment used in such subsea intervention projects may not be readily available and they are much more expensive than their land-based counterparts. Moreover, intervention on a subsea well is much more complex and involved as compared to intervention projects on surface piercing wells. Thus, intervention on subsea wells may be delayed or not performed at all, or the subsea wells may simply be allowed to operate inefficiently.
So-called lightweight intervention was initially introduced in the North Sea in an effort to increase accessibility and reduce the costs associated with intervention of a subsea well. Generally, as shown in FIG. 1, lightweight well intervention involves the use of a relatively smaller work vessel 1 with moderate lifting capacity to go to the offshore site and lower a Lightweight Intervention Package (LIP) 5 on guidelines down to the subsea tree 4 that is coupled to a well 10 at the floor 3. The LIP 5 may include a Lower Riser Package (LRP 5A) 5A (similar to the LRP 5A mentioned above for the completion workover riser), a subsea lubricator 5B and pressure control head (PCH) 5C end connection, provided to accomplish the same intervention jobs with respect to the well, but entering thorough a subsea pressure containment mechanism (subsea lubricator) as opposed to using a surface lubricator and conduit (completion/workover riser) extending from the vessel all the way to the Christmas tree on the sea floor. Also depicted in FIG. 1 is a drum 13 for wireline 9 that is employed to operate or retrieve data from a tool (not shown) to be positioned within the well 10. For subsea lightweight well intervention, the drum 13 and the wireline 9 are used to lower a wireline tool from the surface to the subsea lubricator 5B and to retrieve the wireline tool after the well intervention is completed. The vessel 1 also comprises drums 14 and 17 for umbilical control lines 7 and 16, respectively. The umbilical line 7 may be employed to supply hydraulic and/or electrical power to the LIP equipment (and possibly the subsea Christmas tree) positioned above the subsea tree 10. This umbilical line 7 may also be employed to supply circulating fluids for well control and/or treatment chemicals needed during the well intervention. The umbilical line 16 is employed to control an illustrative remotely operated vehicle (ROV) 15 which may be employed to perform a variety of subsea operations well known to those skilled in the art. One or more extra lines (not shown) from the vessel may be employed to guide various structures or components to the sea floor.
When performing an intervention in a hydrocarbon well it is necessary to isolate the well from the environment. When performing intervention operations on a subsea well 10 using wireline techniques (braided wire, composite cable or slickline), pressure in the well during operations must be contained and structures must be employed to prevent or reduce hydrocarbons from escaping into the surrounding environment. To achieve these objectives, intervention operations involve the use of an isolation control device (lower riser package—LRP) 5A, a pressure control head (PCH) 5C and a lubricator 5B. The PCH 5C provides a dynamic seal between the cable 9 and the wellbore enclosures to maintain pressure control and prevent wellbore fluids from leaking into the environment. The lubricator 5B is a length of pipe that is used to hold a tool during insertion and withdrawal from the well 10, and the isolation control device (LRP) 5A controls the environment between the lubricator 5B and the rest of the well.
At the start of such a process, the LRP 5A and the lubricator 5B are lowered to the well 10 and retrieved therefrom by wire rope (not shown) or by drill pipe. Guidance and alignment of equipment may be done with the use of one or more guidelines (not shown) that are well known to those skilled in the art. After the LRP 5A and subsea lubricator 5B are secured to the subsea Christmas tree 4, a wireline deployed tool (not shown) may be employed to enter the well 10 via a simple wireline 9 intervention. During such a process, the tool is lowered to the well 10 and retrieved therefrom by the same wire 9 that will eventually lower the wireline tool up and down inside the well. The ROV 15 may provide added assistance (in addition to the guidelines) to align the wireline tool with the topmost entry point of the lubricator 5B. Once the tool is safely inside the lubricator cavity, a stuffing box, or pressure control head (PCH) is lowered by a separate wire rope or drill pipe until the PCH 5C reaches the entry point of the lubricator 5B. Then, the PCH's connector is remotely locked and provides a pressure-tight seal over the entry to the lubricator 5B. After connection, the lubricator 5B and stuffing box assembly may be pressure tested to assure proper connection and well control containment. This is followed by opening the valves in the subsea Christmas tree 4 to allow the wireline tool access into the well under a pressure controlled condition. The PCH 5C assembly contains a packing arrangement containing one or more seals which allows the wireline 9 to pass therethrough and the wireline tool is raised or lowered into the well. After the well intervention is completed, the tool is pulled up via the wireline 9 back into the lubricator cavity and the tree and LRP valves are closed. This is followed by evacuating any residual well fluids trapped in the lubricator 5B and replacing them with sea water and the pressure equalized to ambient conditions using a plurality of hydraulic line conduits (leading to the control umbilical) to circulate fluids in and out of the lubricator cavity. Afterwards, the PCH 5C is unlocked from the end of the lubricator 5B and the PCH 5C is hoisted back to the surface vessel.
Normally, the lubricator consists of a number of pipes that are made up together permanently on the vessel to the desired length. The PCH 5C is releasably connected to the top of the lubricator. In normal operations, a BOP is first lowered from the vessel and connected to the top of the Christmas tree 4. The lubricator 5B is lowered and connected to the BOP.
The cable or wireline 9 is inserted through the PCH 5C and then the tool is attached to the cable end. The PCH 5C is arranged so that the cable or wireline 9 can slide through the PCH 5C while the PCH 5C maintains a seal around the cable or wireline 9, as is well known in the art. Now the whole assembly can be lowered towards the well by paying out the cable or wireline from the drum 13 on the vessel. As the assembly reaches the lubricator stack on the seabed, the tool enters into the lubricator 5B. Further lowering of the assembly brings the PCH 5C into contact with the top of the lubricator 5B. A mechanical connector (not shown) is used to releasably lock the PCH 5C and the lubricator 5B together. The tool is now inside the lubricator. To lower the tool into the well, the valves in the BOP are opened and additional cable 9 is played out so that the tool is lowered into the well. The PCH 5C seals around the cable 9 during the operation, thus acting as a barrier against the well pressure.
If multiple wireline interventions are required on the wireline, the wireline tool is redressed or exchanged with another tool at the surface and the tool is redeployed to the subsea lubricator 5B; followed by redeployment/connection of the PCH 5A. If the job is complete, the LRP 5A and lubricator 5B are separately retrieved from the well 10, using wire rope (not shown) or drill pipe.
The use of all of these hoisting wire ropes, guidelines, etc. is envisioned as becoming more difficult as water depth increases. Such an operation with multiple guidelines, equipment and wire rope can be cumbersome. More specifically, handling the wire rope and the multiple guidelines results in fouling problems that the crew of the workover vessel must deal with, to a point where operations become inefficient or more difficult to perform.
The lubricator 5B will behave as a vertical column and be affected by forces of current and other producing bending stresses. The length of the lubricator 5B is limited by tower or crane lifting height restrictions. The length of the lubricator 5B should therefore be as short as possible but must necessarily be longer than the tool to be able to hold the tool before going in or after coming out of the well. This is a limitation that limits the use of riserless well intervention to tools shorter than about 20 meters. However, some operations may require a longer tool than normal tools and today the practice is to use standard riser for operations requiring longer tools. Examples of such tools are perforating guns and straddle packers that may be up to 40 meters long. Since operations involving risers are far more costly than riserless techniques it is desirable to find means of extending the scope of wireline operations. As shown in FIGS. 8 and 9 and described herein, a subsea lubricator stack is depicted as the tool 440 is lowered onto the stack prior to insertion into the well.
The present invention is directed to various devices and methods for solving, or at least reducing the effects of, some or all of the aforementioned problems.